The burner works OK . The blower works OK . What was good heat at the vents is now very cool (not cold ) . Changed the filter.

Depending upon your type of furnace it could be a restricted orifice for the main burner. It could also be low gas pressure if it is a gas unit. You need to post the name on the equiptment with model and the type of fuel being used.

Depending upon your type of furnace it could be a restricted orifice for the main burner. It could also be low gas pressure if it is a gas unit. You need to post the name on the equiptment with model and the type of fuel being used.
I scraped and cleaned the heat exchanger . Now there is no restriction and it heats fine.
*note that it had not been cleaned in many,many years .